FIG. 1 shows a conventional vertical LED. The conventional vertical LED includes a sandwich structure formed by an N-type semiconductor layer 1, a light-emitting layer 2 and a P-type semiconductor layer 3. Below the P-type semiconductor layer 3, a mirror layer 4, a buffer layer 5, a binding layer 6, a silicon substrate 7 and a P-type electrode 8 are disposed in sequence. A surface of the N-type semiconductor layer 1 is processed by a roughening treatment for increasing an optical emission rate. An N-type electrode 9 is further disposed on the roughened surface of the N-type semiconductor layer 1.
By applying a voltage between the N-type electrode 9 and the P-type electrode 8, the N-type semiconductor layer 1 is enabled to provide electrons and the P-type semiconductor layer 3 is enabled to provide holes. Light is produced by the electrons and holes combining at the light-emitting layer 2 to further generate excited light through energy level jump.
Again referring to FIG. 1, a region right below the N-type electrode 9 has a highest current density, and so a main light-emitting region of the light-emitting layer 2 falls right below the N-type electrode 9. However, since the N-type electrode 9 is opaque and occupies 10% to 23% of an overall area of the LED, a substantial ratio of the excited light is shielded. More specifically, the loss of the excited light may reach as high as 15% to 38% due to the main light-emitting region right below the N-type electrode 9, such that light extraction efficiency suffers.
Referring to FIG. 2, to increase the light extraction efficiency, a current blocking layer 3A is often disposed between the P-type semiconductor layer 3 and the mirror layer 4. The current blocking layer 3A is disposed right below the N-type electrode 9, inferring to that a current can only reach the light-emitting layer 2 by first bypassing the current block layer 3A. Thus, the main light-emitting region of the light-emitting layer 2 is no longer situated right below the N-type electrode 9, so that the excited light shielded by the N-type electrode 9 is reduced to increase the light extraction efficiency.
However, the presence of the current block layer 3A leads to increased resistance that reduces light-emitting efficiency of the LED. Further, a transmission speed of a horizontal current is greater than that of a vertical current. When the current block layer 3A is utilized for guiding the current not to pass through the light-emitting layer 2 right below the N-type electrode 9, the light-emitting layer 2 right below the N-type electrode 9 still has considerable brightness due to the transmission of the horizontal current. As a result, a substantial amount of light remains being shielded by the N-type electrode 9 to lead 5% to 20% loss. In conclusion, the light extraction efficiency is limited and fails to fulfill actual requirements.
In another conventional approach for increasing the light extraction efficiency, a thickness of an epitaxy layer is enlarged or a doping concentration is increased to improve a current transmission capability. Through the high current transmission capability, the area of the N-type electrode 9 can be reduced to further decrease a shielded region. However, regardless whether the thickness of the epitaxy layer is enlarged or the doping concentration is increased, a quality of the epitaxy is severely degraded. Further, a current density gets larger as the current increases, such that problems of a boosted forward voltage and burning the electrode are likely incurred.
In yet another conventional approach for increasing the light extraction efficiency, a reflection metal is disposed right below the N-type electrode 9 to reflect the excited light shielded by the N-type electrode 9. It should be noted that, in a vertical LED, as previously stated, the mirror layer 4 is already disposed below the P-type semiconductor layer 3. When the reflection metal is disposed right below the N-type electrode, the excited light is incessantly reflected between the reflection metal and the mirror layer 4 till the excited light is fully depleted. Consequently, the excited light is still not extracted, namely the light extraction efficiency cannot be increased.